Quantitative Analysis of Climate Versus Human Impact on Sediment Yield Since the Lateglacial: the Sarliève Palaeolake Catchment
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Research paper The Holocene 20(4) 497–516 Quantitative analysis of climate versus © The Author(s) 2010 Reprints and permission: sagepub.co.uk/journalsPermissions.nav human impact on sediment yield since DOI: 10.1177/0959683609355181 the Lateglacial: The Sarliève palaeolake http://hol.sagepub.com catchment (France) Jean-Jacques Macaire,1 Agathe Fourmont,1 Jacqueline Argant,2 Jean-Gabriel Bréhéret,1 Florent Hinschberger1 and Frédéric Trément3 Abstract Minimum rates of solid (SSY) and dissolved (DSY) sediment yield (SY) were evaluated in t/km2 per yr from sediments stored in the Sarliève palaeolake (French Massif Central) for seven phases of the Lateglacial and Holocene up to the seventeenth century. The catchment (29 km2), mainly formed of limestones and marls, is located in an area rich in archaeological sites in the Massif Central. The respective impacts of human activities and climate on SY were compared by quantification of human settlements through archaeological survey and palynological data. During the Lateglacial and early Holocene up to about 7500 yr cal. BP, variations in SSY and DSY rates were mainly related to climate change with higher rates during colder periods (Younger Dryas and Preboreal) and lower rates during warmer periods (Bölling-Alleröd and Boreal). However, CF1 tephra fallout induced a sharp increase in SY during the Alleröd. During the middle and late Holocene after 7500 yr cal. BP, SSY and DSY greatly increased (by factors of 6.5 and 2.8, respectively), particularly during the Final Neolithic at about 5300 yr cal. BP when the climate became cooler and more humid. After this date, at least 75% of the SSY increase and more than 90% of the DSY increase resulted from human activities, but SSY rates showed little variation during Protohistoric and Historic Times up to the seventeenth century. SSY and DSY rates and DSY/SSY ratio indicate that catchment soils began to form during the Lateglacial and Preboreal, thickened considerably during the Boreal and Atlantic, finally thinning (rejuvenation) mainly as the result of human-induced erosion during the sub-Boreal and sub-Atlantic. Increased mechanical erosion during the late Holocene also induced an increase in chemical erosion. Keywords archaeological survey, climate-change impact, French Massif Central, Holocene, land-use impact, Lateglacial, palaeolake, soil erosion, solid and dissolved sediment yield Introduction Current earth surface sediment yield, mainly assessed from solid of lake sediments is often difficult to differentiate and quantify. In and dissolved fluxes in rivers, is highly dependent on human a catchment of given physiography, the ratio of dissolved versus activities whose impact is difficult to quantify compared with that solid yield depends considerably on runoff and thus on vegetation of natural factors, particularly climate (e.g. Judson and Ritter, related to climate and recently to human activities (Walling and 1964; Douglas, 1967; Milliman and Syvitski, 1992; Ludwig and Webb, 1986). Probst, 1998). There are three main questions regarding human An overall increasing sediment yield over the last few thou- impact on sediment yield (Hooke, 2000; Ruddiman, 2003; sand years, with some intermittent periods of lower yields, is Wilkinson, 2005). (1) When did it start? (2) How did it evolve? (3) generally interpreted as the result of human activities (e.g. What was its intensity? These questions can be investigated by Zolitschka, 1998; Hooke, 2000; Edwards and Whittington, 2001). assessing pre-human and syn-human sediment yields, through This interpretation is based on various proxies of anthropogenic studying sedimentary archives stored in favourable basins, par- impact. The most usual human-activity proxy is the palynological ticularly lakes (e.g. Campy et al., 1994; Macaire et al., 1997; Dearing and Jones, 2003). Many studies have shown a trend of increasing sediment yield over several thousand years in popu- 1 Université François-Rabelais de Tours, CNRS/INSU, Université lated areas (e.g. Zolitschka, 1998; Bichet et al., 1999; Dearing and d’Orléans, UMR-CNRS 6113 ISTO, France 2Université Claude Bernard Lyon I, UMR 6636, France Jones, 2003). These studies mainly concern solid sediment yield 3Université de Clermont-Ferrand, EA 1001, France deduced from quantification of detrital sediment stores. Dissolved yield has been more rarely quantified (Einsele and Hinderer, 1998; Received 12 April 2009; revised manuscript accepted 24 October 2009 Gay and Macaire, 1999), although chemical erosion has been found to be of major importance in some lithological or biocli- Corresponding author: Jean-Jacques Macaire, Université François-Rabelais de Tours, CNRS/ matic environments (Meybeck, 1987; Berner and Berner, 1987). INSU, Université d’Orléans, UMR 6113 ISTO, Faculté des Sciences et There is very little infor mation about chemical sediment flux as Techniques, Avenue Monge, 37200 Tours, France compared to the detrital fraction because the precipitated fraction Email: [email protected] 498 The Holocene 20(4) Surficial formations A Paris B 400 Puy Sarliève paleolake Loire river lake outlet d'Anzelle sediments 528 m Puy tière riv. de Bane Ar 542 m Alluvial terrace I Basalt-rich 400 slope sediments Carbonate-rich A71 slope sediments Sarliève 400 paleolake Bed-rock formations Clermont- A72 Ferrand 345 m Puy de Thiers Miocene basalts Dôme 500 1465m Miocene detrital sediments 600 II Puy de Oligocene volcano- Gergovie sedimentary formations Sancy Issoire Plateau 1885m 745 m Oligocene limestones and marls 700 N I, II: cross-sections 15 km 600 (see Fig. 2) on riv. Auz 400 Elevation (metres) Sarliève paleolake catchment 1 km 500 Figure 1. Location (A) and main physiographical characteristics (B) of the Sarliève catchment record which provides indirect information (deforestation, cereal INTCAL04 data set. The ‘Marais de Sarliève’ is an ancient lake cultivation, pasture, etc.) from vegetation. Evidence of early (surface area: 5 km2 maximum, altitude: approximately 345 m), human presence is generally provided directly by artefacts, building formed a little before 13 700 yr cal. BP (Fourmont et al., 2006). It structures, etc., but data are often very sparse and were not deter- is located in the Auvergne region, 10 km southwest of Clermont- mined in a systematic manner, and so make it impossible to quan- Ferrand in the Limagne rift (x = 663 780 m; y = 2 082 180 m in tify the density of human occupation and compare it with sediment the Lambert Conformal Conic system; Figure 1A). The origin of yields at different periods. the lacustrine depression is probably tectonic (Ballut, 2000; The aim of this work was thus: (1) to quantify sediment yield Trément et al., 2007b; Fourmont et al., 2009); it can only be (SY) from lacustrine sediment stores for different periods before drained by evaporation or via the outlet located at the northeastern and during human impact since the Lateglacial, distinguishing end of the catchment (Figure 1B). The depression filled with rates of solid sediment yield (SSY) from dissolved sediment yield sediments until it was drained by humans during the seventeenth (DSY), (2) to compare these rates with quantitative data of human century (Fournier, 1996); it is bordered on the north and east by settlement (age, density and location of archaeological sites) from alluvial terraces. The catchment (total surface area: 29 km2) is detailed and systematic archaeological surveys in the lake catch- mainly composed of Oligocene marls and limestones, partly dolo- ment; intensity and type of human activities were evaluated from mitic and sometimes gypsum-rich, with small outcrops of basaltic analysis of pollen grains in lacustrine sediments, and (3) finally to rocks on the surrounding hills up to 745 m (Jeambrun et al., 1973). evaluate the land surface lowering by catchment-soil erosion Bed-rock is generally covered with superficial formations derived which contributed to the sediment yield. from Oligocene carbonated rock (Figure 1B). This area is cur- The studied area is a small lacustrine catchment in the centre of rently characterized by a strong continental climate: mean annual France. Lacustrine deposits, stored since the Lateglacial, have rainfall is less than 600 mm and mean annual temperature is been the subject of detailed sedimentological, palaeohydrological approximately 11°C (min 4°C, max 21°C) (Kessler and and geophysical studies (Bréhéret et al., 2003, 2008; Fourmont, Chambraud, 1986). 2005; Fourmont et al., 2006, 2009; Hinschberger et al., 2006). This Lacustrine sediments, studied through deep pits and core drill- catchment, located in a major site of French history (‘oppidum de ings, consist of delta and basin deposits (Bréhéret et al., 2003, Gergovie’), contains considerable evidence of human settlement 2008; Fourmont et al., 2006, 2009). Delta deposits, 4–5 m thick since the Neolithic and has been the subject of systematic archaeo- (D1 to D7, Figure 2A), are located in the median part of the logical surveys (Trément et al., 2005, 2006, 2007a, b). depression and comprise several units. From base to top these are: D1, beige to greenish carbonated clayey-silts (CS); T1, black pyroclastic sands; D2, several metres-thick deltaic sands com- The Sarliève palaeolake and posed of alternating layers of dark tephric sand and light-coloured its catchment: geological more carbonated sandy-silt; D3, sandy colluvium topped by a setting, sedimentological and palaeosoil; D4, ochre sands; D5, homogeneous greenish calcitic palaeohydrological evolution CS including a thin, pale pink tephra T2; D6, charcoal-rich black calcitic CS; D7, greenish-grey to brown calcitic CS. Basin depos- Here, and in the following sections, all ages are given in calibrated its, 5–6 m thick, observed in the north and south lobes of the years BP. Calibration of radiocarbon ages is based on the depression (B1 to B6, Figure 2A, B), are made up of clayey-silty Macaire et al. 499 A - Cross-section I N-NW Basin area Delta area Basin area S-SE Elevation (m) D6 S23 D7D8 B5 B6 S2 SP4 SP3 SP1 S14 S9 S27 S15 344 C D4-T2-D5 D3 342 B4 P D2 340 B3 D1 B2 B1 400 m T1 338 F - Ca.